The present invention relates generally to veneer-based products such as plywood products, laminated veneer lumber (LVL), and parallel strand lumber (PSL). More particularly the present invention relates to the method of manufacture, and products made therefrom, thereof of using a filler having high thermal conductivity, such as metals, carbides, nitrides and carbons in the manufacture of such plywood boards and panels.
Plywood is a panel product manufactured by gluing together one or more veneers to both sides of a veneer, solid wood, or reconstituted wood core. In the case of solid-wood-core plywood and reconstituted-wood-core plywood, an additional intermediate step is the production of cores, which are made by lateral gluing of blocks or strips of wood or by gluing oriented wood chips or flakes with resin adhesives. Plywood has many advantages over natural wood, an important one being greater dimensional stability. Its uniformity of strength, resistance to splitting, panel form, and decorative value make it adaptable to various uses.
In plywood, both structural and decorative plywood, the grain of alternate layers is typically crossed, in general at right angles; species, thickness, and grain direction of each layer are matched with those of their opposite number on the other side of the core. The total number of layers of veneers is typically odd (three, five, or more) so that the panel is balanced around its central axis. This makes plywood stable and less likely to cup, warp, shrink or swell. Thus assembled, the panels are brought to presses, most often hot presses, for gluing with either natural (animal, casein, soybean, starch) or more recently synthetic resins, such as phenol-formaldehyde, urea-formaldehyde, tannin-formaldehyde and urethanes. Certain synthetic resins, such as phenol-formaldehyde, properly used, may produce joints more durable than the natural wood itselfxe2x80x94highly resistant to weather, microorganisms, cold, hot and boiling water, and steam and dry heat.
Laminated veneer lumber (LVL) is also produced using the same technology, but unlike plywood, the laminants in LVL are oriented parallel to the longitudinal axis of the panel or beam.
Veneer used for plywood usually varies in thickness from {fraction (1/28)} to xe2x85x9 inch (0.09 to 0.32 centimeters), but both thinner and thicker veneers often are used. Thin veneers have an advantage, especially for the outer or face plies, in that they set up less severe transverse stresses with changes in moisture content thereby reducing warping and surface checking (superficial cracking) of panels. The use of thin veneer also makes valuable wood go farther. Since successively cut layers of thin veneers are similar in appearance, identical areas from adjacent sheets can be matched to make highly symmetrical ornamental patterns. In lumber-core panels, the plies next to the faces of the crossbands usually are thicker than the face plies and largely control the stability of the panel.
As already mentioned earlier, in a typical veneer-based product manufacturing process, thin sheets of veneer are cross-laminated and glued together with a hot press. Generally, a resin is used as an adhesive between the plies or veneers which bonded under heat and pressure in a hot press. In addition to high press temperatures, the use of an external catalyst may also accelerate the resin cure rate to shorten the press times. Usually a dilute acid or an acid salt (ammonium sulfate and chloride) in a 20 to 30% solution are used as external catalysts. Sodium hydroxide, carbonates, and others are also used to promote fast cure of the resins by accelerating the curing of the resin. The external catalyst is generally acceptable only if it will produce shorter press times at lower concentration in the range of 0.25 to 1% by weight of the resin.
Higher pressing temperatures are generally not preferred, at least partly due to higher energy consumption. More importantly, higher press temperatures tend to generate more volatile organic compounds (VOCs) that may adversely affect the environment. Additional equipment may be needed to prevent the escape of such VOCs. Although press times have been shortened dramatically through these approaches, there is still need to develop cost-effective and environmentally friendly approaches to reduce the press times.
The present invention relates to a plywood comprising a plurality, preferably an odd number, of wood layers, with or without a core; a resin binding these plurality of wood layers; and a filler having a high thermal conductivity mixed in with the resin. The resin is preferred to comprise one or more thermoset resin. Examples of preferred resins include, but are not limited to phenolic resins, urea resins, melamine resins, epoxy resins, urethane resins and mixtures thereof.
The present invention also relates to a method for manufacturing plywood comprising: applying an adhesive comprising a resin and a filler having a high thermal conductivity to a plurality of wood layers; stacking the plurality of wood layers with the adhesive; and applying elevated temperature and elevated pressure to the wood layers to form into the plywood.